The present invention relates to a solid state imaging device incorporating an on-chip color filter, a fabrication method thereof, and a camera incorporating the solid state imaging device.
Conventionally, solid state imaging devices which have a photoelectric converter for converting light to electric charges, such as a CCD solid state imaging device, a MOS solid state imaging device, etc., have been used in various image capturing apparatuses, such as video cameras, digital still cameras, facsimile machines, etc.
A known example of these solid state imaging devices is a color solid state imaging device which has a color filter. A conventional color solid state imaging device includes a primary-color filter of red (R), blue (B) and green (G) or a complementary-color filter of cyan (C), magenta (M), yellow (Y) and green (G), which is stacked over the light receiving surface of two-dimensionally arranged light receiving elements of a solid state imaging element. The color filter has a predetermined pattern such that each color segment corresponds to one light receiving element. The color filter stacked over the light receiving surface of the light receiving elements is generally called an “on-chip filter”.
Light entering the color solid state imaging device is not necessarily perpendicular to the light receiving surface of the color solid state imaging device. If light diagonally entering the light receiving surface passes through one color filter segment diagonally to reach a light receiving element corresponding to an adjacent color filter segment, color mixture occurs.
A known structure which overcomes such a color mixture problem is a color solid state imaging device 91 shown in FIG. 19, wherein light-shielding black films 96a to 96c are provided at boundaries (pixel boundaries) of light-receiving pixel regions in which photodiodes (PDs) are placed (see Japanese Laid-Open Patent Publication No. 2-084766). FIG. 19 is a cross-sectional view schematically illustrating the structure of the conventional color solid state imaging device. The color solid state imaging device shown in FIG. 19 is fabricated through the fabrication steps described below.
First, a dyeable resin is applied at the pixel boundaries on the imaging surface of the solid state imaging device 91 and patterned so as to have a predetermined thickness. The patterned resin is dyed with black dye to form first light-shielding films 96a. Then, a dyeable resin is applied to predetermined ones of the regions defined by the first light-shielding films 96a and then patterned and dyed to form first color filter segments (R) 93.
Then, a transparent anti-dyeing film 97 is formed over the light-receiving surface on which the first light-shielding films 96a and the color filter segments (R) 93 have been formed. Thereafter, on the transparent anti-dyeing film 97, a dyeable resin is applied to a predetermined thick and patterned, and the patterned resin is dyed with black dye, whereby second light-shielding films 96b are formed at the pixel boundaries. Then, a dyeable resin is applied to predetermined ones of the regions defined by the second light-shielding films 96b and then patterned and dyed to form second color filter segments (G) 94.
Then, in the same way, a transparent anti-dyeing film 98, third light-shielding films 96c, and third color filter segments (B) 95 are formed. Lastly, a transparent anti-dyeing film 99 is formed as a protection layer.
With the light-shielding black films 96a to 96c formed at the pixel boundaries, for example, the light which has diagonally entered and passed through the color filter segment (B) 95 is interrupted by the light-shielding films 96a to 96c so as not to reach an adjacent light-receiving pixel region (PD portion) 92. With this structure, color mixture which would be caused by diagonal light can be prevented.
The solid state imaging device has a flattening layer, a color filter layer, and a light-collection lens layer over each of light-receiving sections formed on a substrate. Presently, the light-collection lens is formed through a thermal flow process or by lens pattern transfer using a dry etching technique.
According to a lens formation technique disclosed in Japanese Patent No. 2604890, a photosensitive resist is applied over the upper surface of a substrate and heated at the first temperature. The resist is selectively exposed to light to form a pattern. The pattern is decolored by entire-surface exposure. The patterned and decolored resist is thermally deformed at the second temperature and thermally cured at the third temperature which is higher than the second temperature. The refractive index of the photosensitive resist is about 1.6 where the refractive index of air is 1. When a light-collection lens made of this photosensitive resist is used, the amount of collected light in each pixel is increased, and the photosensitivity is approximately doubled, as compared with a case where a light-collection lens is not formed. However, in this method, the photosensitive resist is restricted not only as to the optical characteristics but also as to various other characteristics, such as application characteristics, patterning characteristics, thermal-flow characteristics, thermal resistance, etc. Thus, selection of the material is not easy. In other words, the process accuracy depends on selection of the photosensitive resist.
According to a lens formation technique disclosed in Japanese Patent No. 3158466, a non-photosensitive polyimide material is applied and then thermally cured. Thereafter, a first photoresist is applied over the non-photosensitive material layer and selectively exposed to light such that a portion of the first photoresist layer above an electrode pad portion is removed. A second photoresist is applied over the resultant structure and selectively removed from the regions corresponding to the light-receiving sections. Thereafter, the resultant structure is heated to form a first light-collection lens template. Then, the first light-collection lens template is etched back to be transferred to the first photoresist layer, whereby a second light-collection lens template is formed. Thereafter, the second light-collection lens template is transferred to the non-photosensitive material layer to form a light-collection lens. The material of this light-collection lens is not restricted as to patterning characteristics or thermal-flow characteristics and therefore enjoys a broader range of material selection as compared with the technique disclosed in Japanese Patent No. 2604890.
The structure disclosed in Japanese Laid-Open Patent Publication No. 2-084766 requires a large number of steps, i.e., (1) formation of light-shielding black film; (2) formation of first color filter; (3) formation of anti-dyeing film; (4) formation of light-shielding black film; (5) formation of second color filter; (6) formation of anti-dyeing film; (7) formation of light-shielding black film; (8) formation of third color filter; and (9) formation of protection layer. Further, as the dimensions of each pixel are decreased because of size reduction or an increased number of pixels of a solid state imaging device, formation of a light-shielding black film pattern by lithography becomes more difficult.
In the technique disclosed in Japanese Patent No. 3158466, the non-photosensitive material has to be diluted with a solvent before being applied onto the substrate. Thus, the stability of the material is poor. Further, since the material is diluted with a solvent, the electron density of the non-photosensitive material decreases, and accordingly, the refractive index of the material decreases.